This invention relates to a method of purifying hydrogen-rich gas streams which contain carbon monoxide and more particularly but not exclusively relates to a method of removing carbon monoxide from reformate gas mixtures.
Hydrogen is one of the most important industrial gases. It is used, for example, in ammonia synthesis, methanol synthesis, chemical hydrogenations, metal manufacture, glass processing and fuel cells. In most of these applications, the hydrogen has to be virtually free of any reactive contaminants.
Solid polymer fuel cells use hydrogen as a fuel. These fuel cells can be operated with a pure hydrogen stream or a hydrogen-rich stream. In vehicular applications, in order to avoid problems associated with hydrogen storage on board a vehicle, it can be preferable to produce the hydrogen on board the vehicle from conventional fuels. For example, a hydrogen-rich stream can be produced by reforming a conventional fuel with water and/or air in a fuel reformer. Such a hydrogen-rich stream typically contains hydrogen, CO, CO2, N2, H2O and traces of hydrocarbons or alcohols, (depending on the fuel).
As stated above, fuel reformers often produce small amounts of carbon monoxide (typically 0.5 to 5 mol %). Carbon monoxide is a severe poison to the fuel cell anode catalyst. Extremely small amounts of carbon monoxide have a detrimental effect on the cell voltage and reduce the fuel cell""s power output. Hence, the carbon monoxide concentration needs to be attenuated to very low levels (preferably below 10 ppm) for the hydrogen-rich reformate to be suitable as fuel cell feed.
Fuel cell systems incorporating a fuel reformer require a carbon monoxide clean-up system which can reliably remove virtually all of the carbon monoxide. For vehicular applications, there are the additional requirements that such a system:
works under varying flow conditions (through-put and carbon monoxide concentration);
is compact;
is economically feasible;
introduces only a small pressure drop; and
has a minimal start-up time.
Catalytic carbon monoxide removal options from a hydrogen-rich stream include:
reduction of carbon monoxide with water vapour to carbon dioxide and hydrogen (water-gas shift);
selective methanation of carbon monoxide to methane; and
selective oxidation of carbon monoxide to carbon dioxide.
As currently practised, none of the above three options is ideal on its own for attenuating the carbon monoxide to the desired concentration below 10 ppm.
Water-gas Shift Reaction
The water-gas shift reaction would appear to be the most favourable because it produces hydrogen, which is fuel for the fuel cell. However, for the hydrogen-rich streams generated in fuel reformers it is not possible to reduce the carbon monoxide concentration to levels acceptable for the fuel cell by the water-gas shift reaction. This results from the fact that the water-gas shift reaction is an equilibrium reaction (also called a reversible reaction).
CO+H2O(g)⇄CO2+H2xe2x88x9241.2 kJ/mole CO
The equilibrium constant for this reaction is a function of the concentrations of the reactants and products as well as of the temperature. Whether the equilibrium conversion will be reached depends on the size of the catalyst bed, the catalyst activity, and the reactor operating conditions. The reaction rate of the water-gas shift reaction can generally be                               Rate          ⁢                                    xe2x80x83                        ⁢                          xe2x80x83                                ⁢          CO                =                  A          ·                      EXP            ⁡                          (                                                -                                      E                    a                                                  RT                            )                                ·                                    [              CO              ]                        ⁡                          [                              H                2                            ]                                ·                      (                                          K                eq                            -                                                                    [                                          H                      2                                        ]                                    ⁡                                      [                                          CO                      2                                        ]                                                                                        [                    CO                    ]                                    ⁡                                      [                                                                  H                        2                                            ⁢                      O                                        ]                                                                        )                                              Equation        ⁢                  xe2x80x83                ⁢        A            
where,
A=reaction rate constant
Ea=reaction activation energy
R=gas constant
T=absolute temperature       K    eq    =                                          [                          H              2                        ]                    eq                ⁡                  [                      CO            2                    ]                    eq                                            [            CO            ]                    eq                ⁡                  [                                    H              2                        ⁢            O                    ]                    eq      
FIG. 4 of the accompanying drawings shows the dry CO percentages from a water-gas shift reaction as a function of temperature and water inlet concentration. The dry feed here is 58% H2, 21% CO2, 19.5% N2 and 1.5% CO. This figure demonstrates that at low temperatures and increasing H2O levels in the feed, the CO concentration in the product is predicted to be quite low based on thermodynamics. In practice, however, these low levels will not be reached because the reaction rate becomes extremely slow at low temperatures as it approaches equilibrium. Equation A illustrates that the reaction rate is exponentially dependent on the temperature. The last term in Equation A shows that if the reaction mixture approaches equilibrium concentrations, the reaction rate approaches zero. Hence, practical water-gas shift reactors usually can only bring the CO concentration down to 0.5-1.0 mol % dry. Depending on the reformer technology, and thus on the amount of CO in the reformate product, one or more water-gas shift reactors (or reactor stages) could be used to reduce the CO to levels suitable for further removal by selective oxidation and/or methanation.
Selective Methanation
Two methanation reactions are possible:
CO+3H2xe2x86x92CH4+H2O(g)xe2x88x92206.2 kJ/mole CO
CO2+4H2xe2x86x92CH4+2H2O(g)xe2x88x92163.1 kJ/mole CO2
Selective methanation of carbon monoxide is only an option when the carbon monoxide concentration in the feed stream to the methanation reactor is sufficiently low (4000 to 100 ppm) so that hydrogen losses are minimised. The methanation process has the major disadvantage that for each molecule of carbon monoxide removed, three molecules of hydrogen are lost. Thus, if the initial carbon monoxide concentrations are high then the hydrogen loss will be significant. Moreover, although selective methanation is an exothermic reaction, if the carbon monoxide concentration in the feed stream to the methanator is low the temperature rise in the methanation is not too great. If carbon dioxide is present in the feed stream, it is important that a selective methanation reactor is operated at a temperature low enough to minimise carbon dioxide methanation to methane and water as a side reaction. For these reasons, a methanation reactor either needs to be cooled or requires a carefully controlled feed inlet temperature.
Selective Oxidation
Two oxidation reactions are possible:
CO+xc2xdO2xe2x86x92CO2xe2x88x92283.1 kJ/mole CO
H2+xc2xdO2xe2x86x92H2O(g)xe2x88x92242.0 kJ/mole H2
As shown above, carbon monoxide can be removed from a hydrogen-rich stream by selective oxidation. This selectivity is relatively easy to achieve at high carbon monoxide concentrations. However, at lower carbon monoxide concentrations where hydrogen is present in a large excess, the hydrogen becomes more competitive and the high selectivity for carbon monoxide is lost. Also, the amount of oxygen injected into the system is important as any excess will tend to react with hydrogen. Thus, if carbon monoxide selectivity decreases and oxygen reacts with hydrogen there will not be enough oxygen left to remove all the carbon monoxide. If extra oxygen is used, hydrogen loss can be significant. Moreover, the levels of carbon monoxide produced by a fuel reformer may vary with time. Thus, the amount of oxygen required to remove the carbon monoxide will vary. Furthermore, as the oxidation reaction is highly exothermic, changes in the amount of oxygen will lead to temperature variations in the catalyst bed. The selectivity of the oxidation reaction is highly temperature dependent and an increase in the amount of oxygen added could result in an increase in the final carbon monoxide level. In summary, it is easy to achieve a reduction in carbon monoxide by selective oxidation from 0.1 to 10 vol % to levels between 350 to 750 ppm with little loss of hydrogen. However, in practice it is very difficult to reach very low carbon monoxide concentrations by selective oxidation alone, without excessive loss of hydrogen.
U.S. Pat. No. 5,271,916 describes the selective removal of carbon monoxide from a hydrogen-rich gas stream by oxidation in a multi-stage system. This system includes two or more catalytic reactors with inter-stage heat exchangers. The hydrogen-rich stream is fed to the first reactor together with a pre-determined amount of oxygen or air. During the exothermic oxidation reaction, the catalyst temperature rises and hence the temperature of the exit stream from the first reactor is higher than the temperature of the entry stream for the first reactor. At higher temperatures, the reaction becomes less selective towards carbon monoxide combustion, and so the temperature of the exit gas stream from the first reactor is reduced in a heat exchanger. The product gas from the heat exchanger is then fed to a second oxidation reactor, where again oxygen is introduced to further decrease the concentration of the carbon monoxide.
The required duties of inter-stage heat exchangers in a multi-stage carbon monoxide clean-up system vary over a wide range since the total flows can vary by up to a factor of ten. In order to fulfil its cooling duties in such a system, a conventional heat exchanger would not be appropriate unless the area could be changed as well. The latter is possible, for example having some of the process gas bypassing the heat exchanger. The cooled gas and the bypassed gas would come together before entering the next stage of the multi-stage process. By tuning the amount of bypass gas relative to the gas which is cooled, a wide range of overall duties for the heat exchanger can be achieved. This is a well known principle. However, the size of such heat exchangers, as well as the pressure drop involved, makes them unattractive for the requirements of a compact hydrogen clean-up system for vehicular applications. Alternatively, cooling can be provided in a heat exchanger by evaporating a liquid on one side of the exchanger tubes and extracting heat from the process gas on the other side of the tubes. However, once again it is not an attractive proposition for vehicular applications to have a reservoir with boiling liquid on board a vehicle.
European Patent Specification number 0321739 A2 and Japanese Published Specification number 58-152093 relate to the removal of carbon monoxide in hydrogen-rich gas streams by means of the water-gas shift reaction in which water is a reactant.
European Patent Specification number 0834948 A2 describes a method of reducing the carbon monoxide in a hydrogen-rich gas stream by selective oxidation. The selective oxidation catalyst, applied to an inert carrier and in the form of pellets, is cooled by supplying water directly onto the pellets thereby regulating the temperature of the catalyst particles.
One object of the present invention is to provide an improved and simplified carbon monoxide clean-up system for vehicular fuel cell systems which is compact, simple to assemble and is capable of dealing with varying flow rates of hydrogen-rich gas and varying levels of carbon monoxide.
Accordingly, the present invention provides a process for purifying a gas stream containing hydrogen and carbon monoxide which process comprises a catalysed reaction for the selective removal of carbon monoxide from the gas stream wherein a controlled amount of liquid water is introduced into the gas stream prior to the catalysed reaction so as to lower the temperature of the gas stream to a predetermined value at which preferential removal of carbon monoxide takes place in the catalysed reaction.
Suitably, the liquid water introduced into the gas stream is mixed with the gas stream and vaporised prior to the catalysed reaction.
Conveniently, the water vaporisation and mixing with the gas stream is enhanced by placing gas distribution means in the flow path of the mixture of water and the gas stream.
Preferably, the gas distribution means comprises at least one of an inert packing material, one or more static mixers and one or more baffles. Further preferably the inert packing material comprises glass beads, ceramic pellets or metal wool or mesh.
The catalysed removal of carbon monoxide can be by selective oxidation, selective methanation or combinations thereof.
Suitably, the catalysed reaction comprises a selective oxidation reaction and air or oxygen is introduced into the gas stream prior to the catalysed reaction.
Preferably, air or oxygen is fully mixed with the gas stream prior to the catalysed reaction.
Further preferably, air or oxygen is introduced into the gas stream either before or at the same point where water is introduced into the gas stream so as to be mixed simultaneously with the water and the gas stream.
Alternatively, air or oxygen is added to the gas stream after introduction of water to the gas stream.
The process of the invention may comprise two or more catalysed reactions for the selective removal of carbon monoxide from the gas stream and liquid water may be introduced to only some of the catalysed reactions.
Depending on the gas stream temperature and composition, the system may not require water injection before every stage. Since the various catalytic stages may have different catalysts particularly well suited to reduce the CO in a certain CO concentration range by either selective oxidation or methanation, the gas outlet temperature of one stage may be the preferred gas inlet temperature of the following stage. Similarly, the temperature of the reformate coming from a reformer or water-gas shift reactor may be well suited for the first catalytic stage. Moreover, some of the catalyst stages may be designed to operate at a certain temperature so as to eliminate one or more cooling stages by water addition.
Depending on the destination of the hydrogen-rich product from the hydrogen clean-up unit, the hydrogen-rich product may be cooled by yet another water injection stage after the last hydrogen clean-up stage. For example, if the hydrogen-rich stream is the fuel for a solid polymer fuel cell, the preferred inlet temperature of the latter is typically in the range 60-90xc2x0 C. Generally, the fuel cell benefits by operation on a fully humidified fuel. Especially, when the fuel cell is operated at lower pressures (1-2 bars), the reformate fuel may not be saturated as it exits the last hydrogen purification stage. Hence, another water injection stage to cool as well as further humidify the hydrogen-rich stream may also be included in the process of present invention.
Fully vaporising and mixing the injected water with the gas stream prior to the associated catalyst bed minimises temperature differences in the catalyst bed in the radial direction and results overall in a more even temperature profile, enhancing the selectivity of the catalysed reaction. If the injected water is not fully vaporised, liquid water may hit the catalyst. Upon vaporisation from the catalyst, this is likely to cause considerable thermal stress to the catalyst pellets or monolith, eventually resulting in catalyst degradation and de-activation.
Where selective oxidation is involved, the added air is also fully mixed with the gas stream before the associated catalyst bed. As with the water, if the air is not well mixed with the gas stream significant temperature differences may occur in the catalyst bed. Part of the bed may receive gas stream with a higher oxygen level, and the temperature rise will be higher while the selectivity will be lower. Other parts of the catalyst bed may receive gas stream with lower oxygen concentrations. This may locally extinguish the reaction, causing the gas stream to pass through the bed without conversion of carbon monoxide.
Depending on the gas stream temperature and as indicated above, the air can be introduced before or at the same place as where the water is injected. In this arrangement the air and water will be mixed simultaneously with the gas stream. However, if the gas stream is too high in temperature ( greater than 300xc2x0 C., depending on the gas stream composition) the introduction of air may initiate gas phase combustion. The latter is very undesirable because of explosion danger and the non-selectivity of the process. If the gas stream is too high in temperature it is important to introduce the air downstream of the water-cooling stage. Another static mixing unit may be necessary to mix the air and gas stream prior to the catalyst stage.
Another way to enhance the mixing of the air or oxygen with the gas stream is by introducing it to the gas stream in a tube with multiple holes. The air or oxygen will then enter the system throughout the width/height of the unit.
Another way to enhance mixing of air or oxygen and water with the gas stream can be accomplished by introducing the water and air or oxygen both through a single injector (eg a two-fluid nozzle).
In one embodiment of the invention the gas stream is a reformate gas mixture.
Preferably, the catalyst for each catalysed reaction is a supported noble metal or base metal catalyst suitable for that reaction.
Suitably, the concentration of carbon monoxide in the gas stream is reduced to below 10 ppm.
Suitably, the process of the invention can be operated in combination with a water-gas shift reaction for the reduction of carbon monoxide in the hydrogen-containing gas stream.
The present invention also provides a multi-bed catalyst system for the purification of a gas stream containing hydrogen and carbon monoxide which operates in accordance with the process as claimed herein.
From another aspect the present invention is apparatus for purifying a hydrogen-rich gas stream which contains carbon monoxide comprising a catalysed reaction zone in which carbon monoxide is selectively removed from the gas stream and means for introducing a controlled amount of liquid water into the gas stream prior to the catalysed reaction zone so as to lower the temperature of the gas stream to a predetermined value at which the preferential removal of carbon monoxide from the gas stream takes place in the catalyst reaction zone.
Preferably, the apparatus is provided with means for mixing the liquid water with the gas stream and vaporising the liquid water prior to the catalysed reaction zone.
Further preferably, the apparatus is provided with gas distribution means located in the flow path of the mixture of liquid water and the gas stream in order to enhance mixing of the liquid water and the gas stream and vaporisation of the liquid water.
Conveniently, the gas distribution means comprises at least one of an inert packing material, one or more static mixers and one or more baffles.
Suitably, the inert packing material comprises glass beads, ceramic pellets, or metal or wool mesh.
In one embodiment, the catalysed reaction zone contains a catalyst for the selective oxidation of carbon monoxide.
In said embodiment, the apparatus is provided with means for introducing air or oxygen into the gas stream prior to the catalysed reaction zone.
Preferably, air or oxygen is fully mixed with the gas stream prior to the catalysed reaction zone.
Further preferably, air or oxygen is introduced into the gas stream either before or at the same point where water is introduced into the gas stream so as to be mixed simultaneously with the water and the gas stream.
Alternatively, air or oxygen may be added to the gas stream after introduction of water to the gas stream.
In another embodiment, the catalysed reaction zone contains a catalyst for the selective methanation of carbon monoxide.
From another aspect, the present invention is a multi-stage hydrogen clean-up system comprising two or more units of apparatus as described above, all of said units being based on selective oxidation of carbon monoxide or all of said units being based on selective methanation of carbon monoxide or said units being a combination of selective oxidation and selective methanation units.
From yet another aspect, the present invention is an on-board fuel cell system for vehicular application comprising (a) a hydrogen production unit; (b) a hydrogen clean-up unit and (c) a fuel cell wherein the hydrogen clean-up unit comprises apparatus as described above.
In addition to fuel cells, the process and apparatus of the invention can be used for the purification of hydrogen in many other industrial applications, eg ammonia synthesis, chemical hydrogenations etc.
The number of stages required to reduce carbon monoxide from a hydrogen-rich stream to levels acceptable for fuel cells depends on the composition of the hydrogen-rich stream, in particular the carbon monoxide content but other gas constituents may also affect the conversion and selectivity. Furthermore, the choice of catalyst can have an impact on the number of reaction stages. Both the activity and the selectivity of a catalyst for carbon monoxide oxidation, for example, can influence the carbon monoxide conversion that can be attained in a particular stage. There is no limit on the number of stages that can be used in the process of the present invention. Each stage can consist of a different catalyst aimed at performing a different reaction, eg selective oxidation, selective methanation or combinations thereof. As stated above, before each stage, the catalyst temperature is decreased by adding liquid water to process stream, which will decrease the temperature of the process stream upon evaporation. The amount of liquid water introduced to the gas steam can be controlled by measurement of the gas stream temperature after the water introduction. Preferably, a thermo couple is placed in the gas stream after the water injection but before the catalyst bed. The temperature read-out can then be compared to a suitable set-point, and the water addition can be adjusted accordingly.